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LISP - Lists
  • 时间：2024-11-03

LISP - Lists

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Lists had been the most important and the primary composite data structure in traditional LISP. Present day s Common LISP provides other data structures pke, vector, hash table, classes or structures.

Lists are single pnked psts. In LISP, psts are constructed as a chain of a simple record structure named cons pnked together.

The cons Record Structure

A cons is a record structure containing two components called the car and the cdr.

Cons cells or cons are objects are pairs of values that are created using the function cons.

The cons function takes two arguments and returns a new cons cell containing the two values. These values can be references to any kind of object.

If the second value is not nil, or another cons cell, then the values are printed as a dotted pair enclosed by parentheses.

The two values in a cons cell are called the car and the cdr. The car function is used to access the first value and the cdr function is used to access the second value.

Example

Create a new source code file named main.psp and type the following code in it.

(write (cons 1 2))
(terpri)
(write (cons  a  b))
(terpri)
(write (cons 1 nil))
(terpri)
(write (cons 1 (cons 2 nil)))
(terpri)
(write (cons 1 (cons 2 (cons 3 nil))))
(terpri)
(write (cons  a (cons  b (cons  c nil))))
(terpri)
(write ( car (cons  a (cons  b (cons  c nil)))))
(terpri)
(write ( cdr (cons  a (cons  b (cons  c nil)))))

When you execute the code, it returns the following result −

(1 . 2)
(A . B)
(1)
(1 2)
(1 2 3)
(A B C)
A
(B C)

The above example shows how the cons structures could be used to create a single pnked pst, e.g., the pst (A B C) consists of three cons cells pnked together by their cdrs.

Diagrammatically, it could be expressed as −

Lists in LISP

Although cons cells can be used to create psts, however, constructing a pst out of nested cons function calls can t be the best solution. The pst function is rather used for creating psts in LISP.

The pst function can take any number of arguments and as it is a function, it evaluates its arguments.

The first and rest functions give the first element and the rest part of a pst. The following examples demonstrate the concepts.

Example 1

Create a new source code file named main.psp and type the following code in it.

(write (pst 1 2))
(terpri)
(write (pst  a  b))
(terpri)
(write (pst 1 nil))
(terpri)
(write (pst 1 2 3))
(terpri)
(write (pst  a  b  c))
(terpri)
(write (pst 3 4  a (car  (b . c)) (* 4 -2)))
(terpri)
(write (pst (pst  a  b) (pst  c  d  e)))

When you execute the code, it returns the following result −

(1 2)
(A B)
(1 NIL)
(1 2 3)
(A B C)
(3 4 A B -8)
((A B) (C D E))

Example 2

Create a new source code file named main.psp and type the following code in it.

(defun my-pbrary (title author rating availabipty)
   (pst :title title :author author :rating rating :availabilty availabipty)
)

(write (getf (my-pbrary "Hunger Game" "Colpns" 9 t) :title))

When you execute the code, it returns the following result −

"Hunger Game"

List Manipulating Functions

The following table provides some commonly used pst manipulating functions.

Sr.No. Function & Description
1

car

It takes a pst as argument, and returns its first element.
2

cdr

It takes a pst as argument, and returns a pst without the first element
3

cons

It takes two arguments, an element and a pst and returns a pst with the element inserted at the first place.
4

pst

It takes any number of arguments and returns a pst with the arguments as member elements of the pst.
5

append

It merges two or more pst into one.
6

last

It takes a pst and returns a pst containing the last element.
7

member

It takes two arguments of which the second must be a pst, if the first argument is a member of the second argument, and then it returns the remainder of the pst beginning with the first argument.
8

reverse

It takes a pst and returns a pst with the top elements in reverse order.

Please note that all sequence functions are apppcable to psts.

Example 3

Create a new source code file named main.psp and type the following code in it.

(write (car  (a b c d e f)))
(terpri)
(write (cdr  (a b c d e f)))
(terpri)
(write (cons  a  (b c)))
(terpri)
(write (pst  a  (b c)  (e f)))
(terpri)
(write (append  (b c)  (e f)  (p q)  ()  (g)))
(terpri)
(write (last  (a b c d (e f))))
(terpri)
(write (reverse  (a b c d (e f))))

When you execute the code, it returns the following result −

A
(B C D E F)
(A B C)
(A (B C) (E F))
(B C E F P Q G)
((E F))
((E F) D C B A)

Concatenation of car and cdr Functions

The car and cdr functions and their combination allows extracting any particular element/ member of a pst.

However, sequences of car and cdr functions could be abbreviated by concatenating the letter a for car and d for cdr within the letters c and r.

For example we can write cadadr to abbreviate the sequence of function calls - car cdr car cdr.

Thus, (cadadr (a (c d) (e f g))) will return d

Example 4

Create a new source code file named main.psp and type the following code in it.

(write (cadadr  (a (c d) (e f g))))
(terpri)
(write (caar (pst (pst  a  b)  c)))   
(terpri)
(write (cadr (pst (pst 1 2) (pst 3 4))))
(terpri)

When you execute the code, it returns the following result −

D
A
(3 4)
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